Field of the Invention
The present invention relates to a method of producing electrolytic manganese dioxide and the electrolytic manganese dioxide produced therefrom, and more specifically, a method of double milling electrolytic manganese dioxide to modify the particle distribution in order to provide higher compact density.
Description of Related Art
Electrolytic manganese dioxide is commonly used as an active material for dry battery cells because it is an inexpensive and abundant material, and it provides excellent discharge and long-term storage performance. For example, electrolytic manganese dioxide is used as a material in the positive electrode of a primary alkaline battery. Electrolytic manganese dioxide is typically prepared by passing a direct current through a plating cell containing an acidic solution of manganese sulfate and sulfuric acid. The positive electrode of this plating cell may include a plate of titanium onto which the electrolytic manganese dioxide is deposited. The negative electrode may be made of graphite or copper or a similar material. The deposited electrolytic manganese dioxide is mechanically removed from the titanium plate after it has reached a thickness of about 1 mm to about 75 mm and may be crushed to a maximum dimension of 25-100 mm, resulting in electrolytic manganese dioxide pieces for further processing. Electrolytic manganese dioxide pieces are further reduced in size to meet the requirements of the battery manufacturer using a grinding or milling process.
Because the electrolytic manganese dioxide is prepared in an acidic bath, the preparation of the electrolytic manganese dioxide generally requires a washing and/or caustic treatment of the electrolytic manganese dioxide to neutralize the residual acidity of the bath. This treatment may be performed before or after the milling of the electrolytic manganese dioxide. If the neutralization step occurs after the milling step, then the electrolytic manganese dioxide particles are typically suspended in an aqueous solution to which sodium hydroxide is added followed by dewatering to separate the aqueous solution from the solid electrolytic manganese dioxide particles once the acidity has been neutralized. The resulting material after this neutralization step is referred to as neutralized electrolytic manganese dioxide.
In a final step, the electrolytic manganese dioxide is dried to certain specifications. For example, for primary alkaline-battery applications, the drying step is generally mild, leaving behind physisorbed water, which may range from about 1% to about 3% of the product weight. The resulting material is the active material for primary alkaline batteries.
Battery manufacturers use the electrolytic manganese dioxide as an active material of the positive electrode in alkaline cells against a zinc anode. The electrolytic manganese dioxide is combined with other materials that make up the positive-electrode precursor, which is compacted by tools. The quantity of electrolytic manganese dioxide packed in a battery of a predetermined volume is an important factor that determines battery performance. Electrolytic manganese dioxide compact density is a parameter used to predict the quantity of electrolytic manganese dioxide which can be packed into a battery. High compact density is preferred by battery manufacturers because a battery having a given volume will have better performance when the electrolytic manganese dioxide is more densely compacted.